Well that could make for some pretty good story if blizzard decides to become half decent at writing.
This expansion should end with the Alliance winning the war handily, the Horde losing most of its leaders. But Anduin does what Anduin does, and spares the remnants of the Horde. The Alliance splinters as a result, no longer having that strong bond, and really having no reason to stick together.
While the Night elves reclaim Kalimdor, they hunt down the remaining soldiers of the Horde, shutting them out of almost all of Northern Kalimdor(against the wishes of Anduin).
The Horde would slowly rebuild itself, do some heavy self reflecting and remember what it was founded on. Gaining new leadership and a resolve stronger than ever, striving for peace with the Alliance instead.
The only stump here is that it would be hard to say how the Alliance would react to this. By this point, the Kaldorei would want nothing to do with anyone, and anyone following their lead would probably turn their backs on Anduin.
Idk for me this would be a great ending. It’d bring some long needed justice for the Kaldorei, a solid story that isn’t a villain bat for the Horde, and some interesting conflict for the Alliance.